The mainstream of a current X-ray CT apparatus is an R/R-type (third-generation) CT apparatus which includes an X-ray source and an arc-shaped detector oriented toward the focal point of this X-ray source disposed on the opposite side of a subject to be examined to the X-ray source. X-rays from the X-ray source are collimated, formed into a fan-shaped X-ray beam and irradiated onto the image-taking cross section of the subject. An image-taking operation is carried out by revolving and measuring transmitted X-rays attenuated by the subject. A measuring operation is carried out at angle intervals of approximately 0.1 to 0.5 degrees of the revolution to obtain projection data of, for example, a total of 600 to 1200 channels.
The detector is made up of many detector elements and outputs of the respective elements are collected as digital data by a measuring circuit to constitute data (view) corresponding in number to the elements for each measuring angle. This view data is transferred successively from a revolving system to a stationary system through a transmission path. The transferred measured data is subjected to preprocessing such as characteristic correction of detection elements, beam correction or log conversion, etc., by an image processor in the stationary system and then reconstructed as a tomogram according to a publicly known algorithm such as a filter correction back projection method.
As one application example of such an X-ray CT apparatus, helical CT is known which enables high-speed inspection by carrying out measurements while moving a table on which a subject is laid simultaneously with revolutions of an X-ray source and detector. In such helical CT which helically scans the subject, acquiring a tomogram of a specific cross section requires the data of the cross section to be obtained from the helically obtained data through interpolation and such an interpolation processing technique is disclosed in, for example, in U.S. Pat. No. 4,789,929. Applying interpolation processing makes it possible to reduce artifacts due to movements.
Furthermore, multi-slice CT is available which divides the detector into a plurality of arrays and enables projection data of a plurality of cross sections to be measured simultaneously. The multi-slice CT simultaneously collects as many views as arrays, and can thereby take tomographic images of a plurality of cross sections in the case of normal table fixed scanning.
When a helical scanning is carried out using this multi-slice CT, it is necessary to carry out interpolation processing as with a single slice or reconstruct slices with weights equivalent thereto assigned.
U.S. Pat. No. 5,541,970 discloses a case of constructing weighting factors to achieve interpolation with the closest opposite beam and realizing helical correction. On the other hand, Japanese Patent Application Publication No. 9-285460 proposes a method for enhancing continuity by smoothing weighting factors in the Z-axis direction. However, these conventional multi-slice CT apparatuses do not have expandability in their helical scanning such that changes in the relationship between the number of detector arrays and helical pitch are not handled, and interpolation dimensions are not increased.
Therefore, the inventors of this patent application have disclosed a multi-slice X-ray CT apparatus to solve the above described problems in International Publication WO01/28425. While moving a subject in the direction of the body axis and rotating an X-ray source and detector arrays, this CT apparatus carries out a helical scanning for measuring X-rays which have passed through the subject, estimates a virtual detector array to complement the number of detector arrays when the helical pitch (ratio of the distance the subject moves when the X-ray source and the detector array make one rotation to the distance between the detector arrays) is greater than the number of detector arrays, distributes weights set for this virtual detector array to the weights of projection data of a real detector used to obtain the projection data of the virtual detector array. This CT apparatus is supposed to reconstruct a tomogram using projection data obtained from all the detectors of the plurality of detector arrays. Consequently, the number of the detector arrays is fixed and the relationship between the helical pitch and the number of the detector arrays is not optimized, and therefore many artifacts occur and the image quality characteristic changes irregularly when the helical pitch is changed (that is, the image quality characteristic does not change linearly with respect to the helical pitch). Especially when the number of detector arrays used for measurements and the helical pitch are the same, the image quality deteriorates a great deal. Furthermore, strong artifacts may be generated due to discontinuity of projection data at a specific phase (specific rotation angle of the X-ray source focus). Thus, in the X-ray CT apparatus disclosed in above described WO01/28425, the image quality characteristic changes not linearly but irregularly with respect to the helical pitch despite the fact that a weighting function is used according to the same rules for the number of detector arrays and helical pitch. This is attributable to the fact that the number of detector arrays used is constant all the time. If the image quality characteristic changes irregularly with respect to the helical pitch, it is not possible to adjust the helical pitch according to the desired image quality (intensity of artifacts). That is, it is more difficult to optimize the helical pitch for acquiring the desired image quality.
The present invention has been implemented in view of such circumstances and it is an object of the present invention to provide a multi-array detector X-ray CT apparatus and a method for creating a tomogram capable of suppressing artifacts and acquiring a good image.